spectroscopy

• Types of NMR


  • C13 NMR
  • H (proton) NMR
• There is only around 1% C13 in organic molecules but modern NMR machines are sensitive enough to give a full spectra for C13
• C13 NMR spectrum


  Simpler spectrum than the H NMR
• Equivalent Carbon atoms


  In a C13 NMR spectrum, there is one signal (peak) for each set of equivalent C atoms
• C13 NMR spectra


  • 1,4 dinitrobenzene: 3 peaks
  • 1,3 dinitrobenzene: 4 peaks
  • 1,2 dinitrobenzene: 4 peaks
  • CH3NO2: 2 peaks
  • CH3Cl: 2 peaks
  • CH3CH3: 1 peak
• Equivalent Hydrogen atoms


  In an H NMR spectrum, there is one signal for each set of equivalent H atoms. The intensity (integration value) of each signal is proportional to the number of equivalent H atoms it represents.
• H NMR spectra


  • Ethanol: 3 sets of equivalent H's, ratio 3:1:2
  • CH3CH2Br: 4 sets of equivalent H's, ratio 3:1:2:3
  • CH3COOCH3: 3 sets of equivalent H's, ratio 3:2:3
  • CH3CH2CH3: 3 sets of equivalent H's, ratio 3:2:9
  • CH3CH2CH2CH3: 4 sets of equivalent H's, ratio 6:1:2:3
• Samples are dissolved in solvents without any 1H atoms, e.g. CCl4, CDCl3
• A small amount of TMS (tetramethylsilane) is added to the sample to calibrate the spectrum
• Chemical shift (δ)


  A scale used to record the spectra, measured in parts per million (ppm) and is a relative scale of how far the frequency of the proton signal has shifted away from that for TMS
• The δ depends on what other atoms/groups are near the H – more electronegative groups gives a greater shift
• If D2O is added to a sample then a process of proton exchange happens with the H in any O-H and N-H bonds, removing the peaks from the H-NMR spectra
• C13 NMR shift


  • Carbons in different environments have different shifts: 160-220 ppm for C=O, 50-90 ppm for C-O, 0-45 ppm for C-C
• It is not possible to distinguish between similar shifts for each carbon in a benzene ring
• Spin-Spin coupling in H NMR

  Splitting of peak = number of inequivalent H's on neighbouring C atoms + 1
• Splitting patterns


  • Singlet: 1 peak, 0 neighbouring H
  • Doublet: 2 peaks, 1 neighbouring H
  • Triplet: 3 peaks, 2 neighbouring H
  • Quartet: 4 peaks, 3 neighbouring H
  • Quintet: 5 peaks, 4 neighbouring H
• Hydrogens bonded to a nitrogen or oxygen usually do not couple with other protons and appear as singlets on the NMR spectra
• You will not be asked to interpret splitting patterns for the protons attached to a benzene ring
• Use spectra to identify particular functional groups limited to data presented in wavenumber form e.g. an alcohol from an absorption peak of the O–H bond
• Modern breathalysers measure ethanol in the breath by analysis using infrared spectroscopy
• 1. H absorptions tend to be broad
• Infrared spectroscopy


  Certain bonds in a molecule absorb infra-red radiation at characteristic frequencies causing the covalent bonds to vibrate
• Infrared spectra


  • BELOW 1500 cm-1 – "Fingerprinting" - Complicated and contains many signals – picking out functional group signals difficult. This part of the spectrum is unique for every compound, and so can be used as a "fingerprint"
  • ABOVE 1500 cm-1 – "Functional group identification" - Use an IR absorption table provided in exam to deduce presence or absence of particular bonds or functional groups
• Wavenumber ranges for bonds

  • C-O 1000-1300
  • C=O 1640-1750
  • C-H 2850 -3100
  • O-H Carboxylic acids 2500-3300 Very broad
  • N-H 3200-3500
  • O-H Acohols, phenols 3200- 3550 broad
• Mass spectrometry


  Measuring the Mr of an organic molecule. If a molecule is put through a mass spectrometer it will often break up and give a series of peaks caused by the fragments. The peak with the largest m/z, however, will be due to the complete molecule and will be equal to the Mr of the molecule. This peak is called the parent ion or molecular ion.
• Molecular ion formation


  M → [M]+. + e–
• Fragmentation


  [M]+. → X+ + Y.
• Relatively stable ions such as carbocations R+ such as CH3CH2+ and acylium ions [R-C=O]+ are common. The more stable the ion, the greater the peak intensity.
• Elemental analysis


  Work out empirical formula

  2. Using molecular ion peak m/z value from mass spectrum calculate molecular formula

  3. Use IR spectra to identify main bonds/functional group

  4. Use NMR spectra to give details of carbon chain
• C8H16O2 could be an ester, carboxylic acid or combination of alcohol and carbonyl. Look for IR spectra for C=O and O-H bonds
• There is a C=O but no O-H absorptions, so must be an ester
• 4 peaks in NMR - only 4 different environments
• Singlet of area 9 at δ =0.9 means 3 CH3 groups
• Peak at δ 4 shows H–C–O, Area 2 suggests CH2, Quartet means next to a CH3
• Peak at δ 2.2 shows H–C=O, Area 2 suggests CH2, Singlet means adjacent to C with no hydrogens
• Peak at δ 1.2 shows R-CH3, Area 3 means CH3, Triplet means next to a CH2
• Final structure: C CH3 C H3 CH3 CH2 C O O CH2 CH3